The present invention relates to a method of manufacturing a granular building material from refuse.
As is well known, it has become increasingly difficult to solve the problems posed by the necessity of eliminate a constantly increasing amount of refuse and various residues, and thereby also eliminate the risks of pollution and other nuisances.
With the aim of providing an acceptable solution to these problems, it has been proposed to transform refuse, which has been crushed and rid of metallic constituents without a special sorting operation, into solid bodies of various shapes and dimensions which may be employed as building material.
Thus, for example, French Patent No. 2,098,777 describes the manufacture of hard, compact blocks capable of being employed as building or filling material. A metallic silicate is incorporated into a mass of crushed, unsorted refuse. The mass so treated is compacted into blocks under high pressure in a press. The blocks are finally hardened, either at ambient temperature, or by heating, for example, at a temperature from 80.degree. to 200.degree. C.
French Patent No. 2,337,588 describes a method of eliminating household refuse. The method comprises incorporating into a fine, crushed refuse powder rid of metallic bodies, a binder chosen from silicates, various synthetic resins, pitches and lime. Also added are a plasticizer such as powdered cellulose and a substance such as the chloride of sodium, calcium or magnesium, termed the "suitable catalyst for the plasticizer", as well as a mineral filler such as calcium carbonate, quicklime, coal ash, blast furnace flyash, etc. The resultant mixture is subjected to granulation under a high pressure, for example, 50 to 900 bar, so as to form solid granules which may be employed as artificial gravel. This patent mentions that, in order to accelerate the hardening of these granules, they may possibly be heated to a temperature not exceeding 200.degree. C.
French patent application No. 2,472,421 ('421) describes a method of transforming household refuse into artificial gravel. In accordance with the disclosed method, a mass of crushed refuse, rid of metallic bodies, containing at least 30% by weight cellulose, and having a moisture content from 20 to 45% by weight, is caused to react with quicklime and/or a salt of calcium at a temperature lying between 250.degree. and 450.degree. C. The powdery product thus obtained is then agglomerated by compression after the addition of a binder, in order to form granules.
The '421 application likewise describes a variant upon this method in which the granules are subjected to a final heat treatment at a temperature lying between 150.degree. and 350.degree. C. for a length of time sufficient to cause heating at the core.
The '421 application describes a binder for the method which is an aqueous composition containing a mixture of a number of mineral and organic substances including monocalcium phosphate, sulphate of lime, urea, casein, bone glue and polyvinyl acetate as well as cotton fibres and whiting.
On the other hand, the '421 application indicates that various fillers, such as industrial waste, resins and silicates, may be incorporated into the refuse either before crushing it or before agglomeration.
The methods in accordance with the prior art, especially those which are described in the above-mentioned patent documents are not entirely satisfactory, especially because solid bodies are not obtained exhibiting both good mechanical properties and high chemical and biological inertness as well as practically zero solubility in water, as would be desirable for a granular building material.
An aim of the present invention is precisely to provide a method for obtaining granules exhibiting an optimum combination of mechanical and physico-chemical properties particularly well suited for employment as building material, especially as granulate for the preparation of concrete.